The present invention relates to an optical pickup apparatus capable of conducting recording and/or reading (recording/reading) for plural optical information recording media each having a transparent base board differing in terms of thickness from others.
TABLE 1 Thickness Necessary of numerical transparent aperture NA base board (light source Optical disk (mm) wavelength .lambda. nm) CD, CD-R 1.20 0.45 (.lambda. = 780) (reading only) CD-R 1.20 0.50 (.lambda. = 780) (recording and reading) LD 1.25 0.50 (.lambda. = 780) MD 1.20 0.45 (.lambda. = 780) MO 1.20 0.55 (.lambda. = 780) (ISO3.52 inch, 230 MB) MO 1.20 0.55 (.lambda. = 680) (ISO3.52 inch, 640 MB) DVD 0.60 0.60 (.lambda. = 635)
Incidentally, though light source wavelength .lambda. needs to be 780 (nm) for CD-R, other optical disks can employ light sources having wavelengths other than those described in Table 1, and in this case, necessary numerical aperture NA varies depending on light source wavelength .lambda. used. For example, in the case of CD, the necessary numerical aperture NA approximates .lambda. (.mu.m)/1.73 while in the case of DVD, the necessary numerical aperture NA approximates .lambda. (.mu.m)/1.06.
As stated above, various optical disks each having its own size, base board thickness, recording density and wavelength used which differ from others are available on the market now, and there have been proposed optical pickup apparatuses each being capable of handling various optical disks.
As one of them, there has been proposed an optical pickup apparatus wherein there are provided converging optical systems corresponding to different optical disks which are switched depending on the optical disk to be reproduced. This optical pickup apparatus, however, requires plural optical pickup apparatuses, and thereby brings about high cost and requires a driving mechanism for switching optical pickup apparatuses, resulting in a complicated apparatus requiring its high accuracy for switching, which is not preferable.
With a background stated above, there have been made proposals to reproduce plural disks with one converging optical system, and for example, there has been proposed an optical pickup apparatus wherein an image-forming spot for DVD and that for CD are arranged so that each of their wave front aberrations is not higher than 0.01 .lambda. (rms) by optimizing two light sources and sizes of apertures, as described in TOKKAIHEI 8-55363.
As another example, TOKKAIHEI 9-184975 discloses an optical pickup apparatus wherein an objective lens is provided with a slight step which is coaxial with an optical axis of the objective lens. In this optical pickup apparatus, a light flux emitted from a light source is divided into two areas including a first area which is closer to an optical axis than the step and a third area which is outside the step, and a light flux passing through the first area is used for recording/reading of CD and a light flux passing through the first area and the third area is used for recording/reading of DVD, thereby recording/reading for two optical disks including both DVD and CD are performed by one converging optical system (hereinafter the converging optical system is called a 2-area converging optical system and when it is applied to an objective lens, it is called a 2-area objective lens).
In Lecture No. OFA 3-1 at "International symposium on optical memory and optical data storage 1996", there is proposed an optical pickup apparatus wherein a shielding ring which is coaxial with an optical axis of an objective lens is provided. In this pickup apparatus, a light flux emitted from a light source is divided into three areas including a first area which is closer to the optical axis than the shielding ring, a shielding area sectioned by the shielding ring itself, and a third area which is outside the shielding ring, and a light flux passing through the first area is used for recording/reading of CD and a light flux passing through the first area and the third area is used for recording/reading of DVD, thereby recording/reading for two optical disks including both DVD and CD are performed by one converging optical system (hereinafter the converging optical system is called a shielding converging optical system and when it is applied to an objective lens, it is called a shielding objective lens).
The optical pickup apparatus equipped with the 2-area converging optical system or the shielding converging optical system is to make wave front aberration (spherical aberration) to be discontinuous, and thereby to reduce an effect of flare caused by spherical aberration resulting from a thickness difference of a transparent base board between DVD and CD in recording/reading of CD.
In these optical pickup apparatuses, the necessary numerical aperture NA in reading of CD can be as small as it is about 0.367 because one light source with short wavelength (.lambda.=635 nm) is used, therefore, diffraction limit characteristics can be satisfied and there by DVD and CD can be reproduced. In this case, when intending to use two light sources for coping with CD-R in place of CD, necessary numerical aperture NA for reading of CD-R is required to be 0.45, which means that high NA is needed similarly to the occasion to cope with LD or the like.
However, in the 2-area converging optical system, it is necessary that the designed thickness of a base board is made to be thicker and a position of the step is moved to the high NA area, which, in this case, worsens a spot shape in reading of DVD and makes it impossible to reproduce DVD. In the shielding optical system, diffraction limit characteristics can barely be satisfied with necessary numerical aperture NA=0.367, and when a position of the shielding ring is moved further to the higher NA area, diffraction limit characteristics can not be satisfied, and reading is impossible.
Taking the foregoing into consideration, inventors of the invention have proposed, in TOKUGANHEI 9-197076, an optical pickup apparatus wherein an objective lens is provided with at least two steps coaxial with an optical axis of the objective lens. In this optical pickup apparatus, a light flux emitted from a light source is divided into three areas including a first area which is close to the optical axis, a second area which is outside the first area, and a third area which is outside the second area, and a light flux passing through the first area closer to the optical axis and the second area is used for recording/reading of the second optical disk and a light flux passing through the first area and the third area is used for recording/reading of the first optical disk, thereby recording/reading for plural optical disks are performed by one converging optical system (hereinafter the converging optical system is called a 3-area converging optical system, and when it is applied to an objective lens, it is called a 3-area objective lens).
Further, in the optical pickup apparatus described in TOKUGANHEI 8-55363, light sources are arranged so that wave front aberrations of image-forming spots just for DVD and CD may be 0.01 .lambda. or less, and nothing is considered for the distance from the final refracting surface which is one closest to the optical information recording medium in the converging optical system to the transparent base board of the optical disk (hereinafter referred to also as working distance WD). When the DVD in the case of DVD is different from that in the case of CD, when reproducing at least one type of optical disk, it is necessary to let a driving electric current flow constantly through an actuator which moves an objective lens in the converging optical system for focusing. This causes a problem that there is no escape from high power consumption. There is further a problem that an actuator is required to be large compared with an exclusive actuator for DVD or CD, to secure a movable distance for focusing.